it is necessary (for using skype) to use OSS drivers (because microphone in Skype does not work with the standard module snd_hda.ko).
I use Skype 1.2.0.18 from ports.
we unload snd_hda.ko and then we installed OSS of 4.0b016 of from the ports.
after the OSS installation, result:

the result: In this configuration the sound exists in the course somewhere of the first three-five minutes, after which first ceases to work ossxmix (OSS mixer), and after it (even through the pair of minutes) is turned off sound.
the attempts to unload the utilized modules complete unsuccessfully:

Hmm..
1) Did you unload the FreeBSD modules before loading OSS?
2) Does this happen only when Skype 1.2 is used? There are newer versions for OSS (1.4, 2.0.0.68 and 2.0.0.72).
3) What does 'ossmix' report after 'ossxmix' has stopped working?